20.1 Introduction

Reliable delivery of fonts is considered a critical requirement for SVG. Designers
require the ability
to create SVG graphics with whatever fonts they care to use and then
have the same fonts appear
in the end user's browser when viewing an SVG drawing,
even if the given end user hasn't purchased the fonts in question. This parallels
the print world, where the designer uses a given font when authoring a drawing for print,
but when the end user views the same drawing within a magazine the text appears with the correct font.

SVG utilizes the web font facility defined in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2]
as a key mechanism for reliable delivery of font data to end users.
A common scenario is that SVG authoring applications will generate
compressed, subsetted web fonts for all text elements used by
a given SVG document fragment. Typically, the web fonts will be saved in
a location relative to the referencing document.

One disadvantage to CSS2's Webfont facility to date is that CSS2 did not specify
particular font formats that were required to be supported. The result was that
different implementations supported different web font formats, thereby making it difficult
for web site creators to post a single web site that is supported by
a large percentage of installed browsers.

To provide a common font format that will exist in all conforming SVG user agents,
SVG includes elements which allow for fonts to be defined in SVG.

20.2 SVG fonts

20.2.1 Overview of SVG fonts

The purpose of SVG fonts is to allow for delivery of glyph outlines
in display-only environments. SVG fonts that accompany web pages
must
be supported only in browsing and viewing situations. Graphics
editing applications or file translation tools
must
not attempt
convert SVG fonts into system fonts. The intent is that SVG files
be interchangeable between two content creators, but not the SVG fonts
that might accompany these SVG files. Instead,
each content creator will need to license the given font
before being able to successfully edit the SVG file.
The
font-face-name attribute indicates the name of licensed font to use for editing,

SVG fonts contain unhinted font outlines. Because of this, on many
implementations there will be limitations regarding the quality and legibility
of text in small font sizes. For increased quality and legibility
in small font sizes, content creators may want to use an alternate font
technology, such as fonts that ship with operating systems or
an alternate web font format.

Because SVG fonts are expressed using SVG elements and attributes,
in some cases the SVG font will take up more space than if the font
were expressed in a different web font format which was
especially designed for compact expression of font data.
For the fastest delivery of web pages, content creators may want to use an alternate
font technology.

A key value of SVG fonts is guaranteed availability in SVG user agents.
In some situations, it
might
be appropriate for an SVG font to be the
first choice for rendering some text. In other situations, the SVG font
might be an alternate, back-up font in case the first choice
font (perhaps a hinted system font) is not available to a given user.

The characteristics and attributes of SVG fonts correspond closely to
the font characteristics and parameters described in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].

SVG fonts and their associated glyphs do not specify bounding box information.
Because the glyph outlines are expressed as SVG graphics elements,
the implementation has the option to render the glyphs either using standard
graphics calls or by using special-purpose font rendering technology, in which case
any necessary maximum bounding box and overhang calculations can be performed from analysis
of the graphics elements contained within the glyph outlines.

An SVG font can be either
embedded within the SVG document fragment that uses the font or saved as an
external file and referenced via a URI reference.

Here is an example of how you might embed an SVG font inside of an SVG
document:

The style of a font. Takes on the same values as the
'font-style' property, except that a comma-separated
list is permitted. The default value is all.
Animatable: no.

font-variant = "[normal | small-caps] [,[normal | small-caps]]*"

Indication of whether this face is the small-caps variant of a font.
Takes on the same values as the
'font-variant' property, except that a comma-separated
list is permitted. The default value is normal.
Animatable: no.

The range of ISO 10646 characters [UNICODE]
covered by the font. For more information, see the description of the
'unicode-range' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
The default value is U+0-7FFFFFFF.
Animatable: no.

units-per-em = "<number>"

The number of coordinate units on the em square, the size of the design grid
on which glyphs are laid out.
For more information, see the description of the
'units-per-em' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

The vertical stroke angle of the font.
For more information, see the description of the
'slope' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
The default value is 0.
Animatable: no.

cap-height = "<number>"

The height of uppercase glyphs in the font within the font coordinate system.
For more information, see the description of the
'cap-height' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

x-height = "<number>"

The height of lowercase glyphs in the font within the font coordinate system.
For more information, see the description of the
'x-height' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

accent-height = "<number>"

The distance from the baseline to the top of accent characters, measure by a distance
within the font coordinate system.
The default value is the value of the
ascent attribute.
Animatable: no.

ascent = "<number>"

The maximum unaccented height of the font within the font coordinate system.
For more information, see the description of the
'ascent' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

descent = "<number>"

The maximum unaccented depth of the font within the font coordinate system.
For more information, see the description of the
'descent' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

horiz-origin-x = "<number>"

The X-coordinate in the font coordinate system of the origin of a glyph
to be used when drawing horizontally oriented text.
The default value is 0.
Animatable: no.

horiz-origin-y = "<number>"

The Y-coordinate in the font coordinate system of the origin of a glyph
to be used when drawing horizontally oriented text.
The default value is 0.
Animatable: no.

horiz-adv-x = "<number>"

The default horizontal advance after rendering a glyph in horizontal orientation.
Glyph widths
are required to be
positive, even if the glyph is typically
rendered right-to-left, as in Hebrew and Arabic scripts.
Animatable: no.

vert-origin-x = "<number>"

The X-coordinate in the font coordinate system of the origin of a glyph
to be used when drawing vertically oriented text.
The default value is half of the value of attribute
horiz-adv-x.
Animatable: no.

vert-origin-y = "<number>"

The Y-coordinate in the font coordinate system of the origin of a glyph
to be used when drawing vertically oriented text.
The default value is the position specified by the font's
ascent attribute.
Animatable: no.

vert-adv-y = "<number>"

The default vertical advance after rendering a glyph in vertical orientation.
The default value is the sum of the values of attributes
ascent and.
descent.
Animatable: no.

text-bottom = "<number>"

The bottom of the font within the font coordinate system.
For more information, see the description of the
'text-bottom' value for the
'vertical-align' property in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

baseline = "<number>"

The lower baseline of a font within the font coordinate system.
For more information, see the description of the
'baseline' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

centerline = "<number>"

The central baseline of a font within the font coordinate system.
For more information, see the description of the
'centerline' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

mathline = "<number>"

The mathematical baseline of a font within the font coordinate system.
For more information, see the description of the
'mathline' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

ideographic = "<number>"

The ideographic baseline of a font within the font coordinate system.
Animatable: no.

hanging = "<number>"

The hanging baseline of a font within the font coordinate system.
Animatable: no.

topline = "<number>"

The top baseline of a font within the font coordinate system.
For more information, see the description of the
'topline' descriptor in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

text-top = "<number>"

The top of the font within the font coordinate system.
For more information, see the description of the
'text-top' value for the
'vertical-align' property in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

font-face-name = "<string>"

The full name of a particular face of a font family. It typically includes
a variety of non-standardized textual qualifiers or adornments appended to
the font family name.
For more information, see the description of full font names in
the "Cascading Style Sheets (CSS) level 2" specification
[CSS2].
Animatable: no.

underline-position = "<number>"

The ideal position of an underline within the font coordinate system.
Animatable: no.

underline-thickness = "<number>"

The ideal thickness of an underline, expressed as a length within the font coordinate system.
Animatable: no.

strikethrough-position = "<number>"

The ideal position of a strike-through within the font coordinate system.
Animatable: no.

strikethrough-thickness = "<number>"

The ideal thickness of a strike-through, expressed as a length within the font coordinate system.
Animatable: no.

overline-position = "<number>"

The ideal position of an overline within the font coordinate system.
Animatable: no.

overline-thickness = "<number>"

The ideal thickness of an overline, expressed as a length within the font coordinate system.
Animatable: no.

20.2.3 The 'glyph' element

The 'glyph' element defines the graphics
for a given glyph. The coordinate system for the glyph is defined by the various
attributes in the
'font' element.

The contents of a 'glyph' can be any SVG graphics elements.
However, in some implementations, faster font rendering (and possibly improved quality)
might
occur when glyph definitions consist of a single
'path' element.

One or more Unicode characters indicating the sequence of Unicode characters which corresponds
to this glyph. If a character is provided, then this glyph corresponds to
the given Unicode character. If a list of characters is provided, then this
glyph corresponds to the given sequence of Unicode characters. One use of a list of
numbers is for ligatures. For example, if unicode="ffl",
then the given glyph will be used to render
the sequence of characters "f", "f", and "l".
(This could alternatively have been expressed using
character entities, using XML character references
expressed in hexadecimal notation: unicode="&#x66;&#x66;&#x6c;",
or XML character references expressed in decimal notation: unicode="&#102;&#102;&#108;".)
When determining the glyph(s) to draw a given character sequence,
the 'font' element is searched from its
first 'glyph' element to its last in lexical order to see if
the upcoming sequence of Unicode characters to be rendered match the
sequence of Unicode characters specified in the
unicode attribute for the given
'glyph' element.
The first successful match is used.

Note that any occurrences of
'altglyph'
take precedence over the glyph selection rules within an SVG font.
Animatable: no.

glyph-name = "<name> [, <name> ]* "

A name for the glyph. It is recommended that glyph names
be unique across a font. The glyph names can be used
in situations where Unicode character numbers do not provide sufficient information
to access the correct glyph, such as when there are multiple glyphs
per Unicode character. The glyph names can be referenced in
kerning definitions.
Animatable: no.

vert-text-orient = "default | h | v"

When drawing vertical text, indicates whether the given glyph is meant to be drawn
with a vertical or horizontal orientation.
The default value is vertOrient="default",
which indicates that the Unicode character number
determines
the orientation of this glyph.
Animatable: no.

arabic = "initial | medial | terminal | isolated"

For Arabic glyphs, indicates which of the four possible forms this glyph represents.
Animatable: no.

han = "ja | zht | zhs | kor"

For glyphs in the Han range, indicates which of the four possible forms this glyph represents.
Animatable: no.

horiz-adv-x = "<number>"

The horizontal advance after rendering a glyph in horizontal orientation.
The default value is the value of the font's
horizAdvX attribute.
Glyph widths
are required to be
positive, even if the glyph is typically
rendered right-to-left, as in Hebrew and Arabic scripts.
Animatable: no.

vert-adv-y = "<number>"

The vertical advance after rendering a glyph in vertical orientation.
The default value is the value of the font's
vertAdvY attribute.
Animatable: no.

20.2.4 The 'missing-glyph' element

The 'missing-glyph' element defines the graphics
to use if there is an attempt to draw a glyph from a given font and the given glyph
has been defined. The attributes on the 'missing-glyph' element
have the same meaning as the corresponding attributes on the
'glyph' element.

An example of a kerning pair are the letters
"Va", where the typographic result might look better if the letters
"V" and the "a" were rendered slightly closer together.

Right-to-left and bi-directional text in SVG is laid out in a two-step process,
which is described in
Relationship with bi-directionality.
If SVG fonts are used, before kerning is aplied, characters are re-ordered
into left-to-right (or top-to-bottom, for vertical text) visual rendering order.
Kerning from SVG fonts is then applied on pairs of glyphs which are rendered
contiguously. The first glyph in the kerning pair
is the left (or top) glyph in visual rendering order. The second glyph in the
kerning pair is the right (or bottom) glyph in the pair.

For convenience to font designers and to minimize file sizes,
a single 'hkern' and 'vkern'
can define a single kerning adjustment value between one set of glyphs
(e.g., a range of Unicode characters) and another set of
glyphs (e.g., another range of Unicode characters).

The 'hkern' element defines kerning pairs
and adjustment values in the horizontal advance value when drawing pairs of glyphs
which the two glyphs are contiguous and are both rendered horizontally (i.e., side-by-side).
The spacing between characters is reduced by the kerning adjustment. (Negative
kerning adjustments increase the spacing between characters.)

A sequence (comma-separated) of Unicode characters
(refer to the description of the
unicode
attribute to the
'glyph'
element for a description of how to express individual Unicode characters)
and/or unicode ranges
(see description of unicode ranges in [CSS2])
which identify a set of possible first glyphs in the kerning pair.
If a given Unicode character within the set has multiple corresponding
'glyph'
elements (i.e., there are multiple
'glyph'
elements with the same
unicode
attribute value, but different
glyphName
values), then all such glyphs are included in the set.
Comma is the separator character; thus, to kern a comma, specify the comma as
part of a Unicode range or as a glyph name using the
g1 attribute.
The total set of possible first glyphs in the kerning pair is the union of
glyphs specified by the
u1 and
g1 attributes.
Animatable: no.

g1 = "<name> [, <name> ]* "

A sequence (comma-separated) of glyph names (i.e., values that match
glyphName
attributes on
'glyph'
elements) which identify a set of possible first glyphs in the kerning pair.
All glyphs with the given glyph name are included in the set.
The total set of possible first glyphs in the kerning pair is the union of
glyphs specified by the
u1 and
g1 attributes.
Animatable: no.

u2 = "[<number> | <urange>] [, [<number> | <urange>] ]* "

Same as the
u1
attribute, except that
u2
specifies possible second glyphs in the kerning pair.
Animatable: no.

g2 = "<name> [, <name> ]* "

Same as the
g1
attribute, except that
g2
specifies possible second glyphs in the kerning pair.
Animatable: no.

k = "<number>"

The amount to decrease the spacing between the two glyphs in the kerning pair.
The value is in the font coordinate system.
Animatable: no.

At least one each of u1 or g1
and at least one of u2 or g2
must be provided.

The 'vkern' element defines kerning pairs
and adjustment values in the vertical advance value when drawing pairs of glyphs together
when stacked vertically. The spacing between characters is reduced by the kerning adjustment.